Mold for injection-compression molding

ABSTRACT

A mold ( 1 ) for injection-compression molding includes a punch ( 2 ) and a matrix ( 3 ) suitable to close on the punch ( 2 ) to delimit with this an injection chamber ( 10 ) to contain the material to be injected. The matrix ( 3 ) and punch ( 2 ) are axially movable between them with respect to an axial direction (Y-Y) of opening/closing of the mold ( 1 ). The mold has a perimetral ring ( 4 ) slidingly associated to the punch ( 2 ) or the matrix ( 3 ), along the axial direction (Y-Y), suitable to define, together with the matrix ( 3 ) and the punch ( 2 ), the profile of the injection chamber ( 10 ). The perimetral ring ( 4 ) includes an interface profile ( 43 ) engaging with the matrix ( 3 ) or with the punch ( 2 ), equipped with a particular geometry generating in the closing phase of the mold ( 1 ), a compression force of the ring ( 4 ) towards the inside of the mold ( 1 ).

FIELD OF APPLICATION

The present invention relates to a mold for injection-compression molding and a relative molding method.

PRIOR ART

Injection-compression molding differs from the traditional molding process in the injection step, which takes place when the mold is open. The injection-compression process involves an initial step in which the mold has an initial opening before the injection step: this increases the volume of the cavity, which is partially filled by the molten material. The molten material is then compressed by the closing movement of the mold and the mold filling time is completed as a result. An alternative method involves filling the entire mold, and the compression step compensates only for the volumetric shrinkage of the molten material that solidifies. This is followed by the steps of cooling and extraction of the piece. In the injection-compression process, the final distribution in the mold, compaction and compensation of the shrinkage of the material occur by compression, by closing the mold through the movement of the press.

In order to ensure the required tolerances on the mold closing, particularly with regard to thermal deformations induced to the mold by the same process, it is known to use perimetral rings, interposed between the upper portion (matrix) and the lower portion (punch) of the mold.

However, this known solution also has some drawbacks:

-   -   the pressure of the molten material can push the perimetral ring         towards the outside of the mold, thus creating burrs on the         piece;     -   in some geometries, the construction of the perimetral ring is         very expensive if not impossible since the perimetral ring must         follow the geometrical features of the piece (such as wings and         ribs) and in many cases, it creates (together with the matrix         and punch) a triple junction point that is very difficult to         form;     -   the uncontrolled movement of the perimetral ring when opening         the mold may damage (such as scratch) the surface of the piece.

DISCLOSURE OF THE INVENTION

As described above, the prior art solutions have some drawbacks.

In particular, to date, no reliable and cos-effective solutions are known that ensure the correct resting and support of the perimetral ring during the injection of the material into the mold.

Moreover, to date, no perimetral ring solutions are known that allow preventing the presence of visible junction lines on the piece or on the outer edge of the piece.

Moreover, to date, no reliable and cost-effective solutions are known that allow preventing the ejection of the piece at the opening by a system that ensures both the necessary tightness of the closure and the absence of residual forces when opening the mold.

The need to solve the drawbacks cited with reference to the prior art is therefore felt.

DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will appear more clearly from the following description of preferred non-limiting embodiments thereof, in which:

FIGS. 1-6 show schematic views of steps of a molding sequence according to the present invention;

FIG. 7 shows a schematic top view of a perimetral ring according to the present invention;

FIG. 8 shows a schematic view of the hydraulic circuit, integrated in the mold, for moving the perimetral ring;

FIG. 9 shows an enlargement of a detail of FIG. 5, in which a perimetral ring is shown, in an embodiment example according to the present invention, in the closing step (between matrix and punch) of the mold;

FIGS. 10-11 show further embodiment examples of the perimetral ring.

Elements or parts of elements in common o the embodiments described below are referred to with the same reference numerals.

DETAILED DESCRIPTION

With reference to the above figures, reference numeral 1 globally denotes a mold for injection compression molding (ICM).

Mold 1 comprises a punch 2 and a matrix 3 suitable to close on the punch 2 so as to delimit with this an injection chamber 10 to contain the material to be injected. Matrix 3 and punch 2 are axially movable between them with respect to an axial direction Y-Y of opening/closing of mold 1.

As shown in the Figures, mold 1 further comprises a perimetral ring 4, slidingly associated to punch 2 or matrix 3, along the axial direction Y-Y, in which the perimetral ring 4 defines, together with matrix 3 and punch 2, the profile of the injection chamber 10.

Mold 1 is characterised in that the closure between matrix 3 and punch 2 is accomplished using a perimetral ring 4 having a special shape of an interface wall with matrix 3 above, or with punch 2.

The perimetral ring 4 comprises a matrix interface profile 43 intended to engage with matrix 3, and a punch interface profile 42, intended to engage with punch 2.

Advantageously, the matrix interface profile 43 is equipped with a particular geometry suitable to generate a compression force of ring 4 towards the inside of mold 1.

The matrix interface profile 43 is convex, preferably convex polygonal.

In particular, the matrix interface profile 43 comprises an inclined upper thrust wall 443, on which an inclined plane 34 associated with matrix 3 is engaged.

As shown in FIG. 2, in the closing step of mold 1, matrix 3 pushes on the perimetral ring 4, and in particular on the upper thrust wall 443, thus generating a force F, substantially transversal to the axial direction Y-Y and directed towards the inside of mold 1, which allows correctly retaining the perimetral ring 4, thus ensuring a correct support.

The perimetral ring 4 comprises, between the matrix interface profile 43 and the punch interface profile 42, an inner profile 41 which at least partially defines the profile of the injection chamber 10.

The perimetral ring 4 defines, with the inner profile 41, the profile of the injection chamber 10 in all the molding process steps, i.e. in the mold closing step (FIG. 2), in the molten material injection step (FIG. 3), in the compression step (FIG. 4) up to the compression end step (FIG. 5).

The perimetral ring 4 defines a portion of the profile of the injection chamber 10 between matrix 3 and punch 2. Therefore, the perimetral ring 4 defines at least partially the profile of the piece, together with matrix 3 and punch 2.

In particular, the inner profile 41 of the perimetral ring 4 defines at least partially the profile of the piece.

The perimetral ring 4 is interposed between matrix 3 and punch 2: such a solution allows obtaining double junction points on the piece. A double junction point is created in the mold by the coupling of two metal surfaces.

As shown in FIG. 9, a first double junction point (J1) is formed on the piece between punch 2 and perimetral ring 4, and a second double junction point (J2) between perimetral ring 4 and matrix 3. Such a solution therefore prevents the formation of a triple junction point on the piece (between punch, perimetral ring and matrix 3) of very difficult, if not impossible, execution.

In addition, the fact that at least a portion of the perimetral ring 4 defines the profile of the piece allows producing pieces with rounded edges, as shown in FIG. 10. This solution is very important for pieces intended to come into contact with the user's skin.

In addition, the fact that at least a portion of the perimetral ring 4 defines the profile of the piece allows to perform an aesthetic photoengraving or embossing treatment (T pattern in bas-relief made in negative on matrix 3), as shown in FIG. 11. This solution, in which the piece has a wavy surface, is very important for pieces intended for the automotive industry, and in general for pieces intended to have an aesthetic value.

Even more advantageously, the perimetral ring 4 defines at least partially thickness S of piece P, together with punch 2. Such a solution allows effectively hiding the junction points on the piece.

In particular, the inner profile 41 of the perimetral ring 4 defines, together with punch 2, a first double junction point (J1) at thickness S of the piece.

Moreover, the inner profile 41 of the perimetral ring 4 defines, together with matrix 3, a second double junction point (J2) at an outer edge C of the piece.

As shown in FIG. 9, in fact, the first double junction point (J1) is placed at thickness S of the piece, and the second double junction point (J2) is placed at an outer edge C of the piece. The second double junction point (J2) derives from a flattening closure: it is therefore easy to make with good quality, and without being visible in the finished piece (being placed at an outer edge C of the piece). The first double junction point (J1) is instead by its geometric and technological nature difficult if not impossible to hide. However, the double junction point (J1) is also concealed and not visible to the user as it is placed at thickness S of the piece. Advantageously, therefore, there are no visible junction lines on the piece, which is very important for pieces intended to be aesthetic details.

Mold 1 is also characterised in that the closing of the injection chamber 10 takes place on the perimetral ring 4, a ring that defines together with punch 2 thickness S of the piece, so that the juncion line is placed on the thickness (i.e. within thickness S) of the piece to be molded.

Mold 1 is operated by axial movement means (not shown) of matrix 3 and/or punch 2 to accomplish the opening and closing of the mold, hereinafter referred to as mold movement means.

Extraction mean 9 (not shown) act on mold 1 adapted to extract the piece from the mold.

Mold 1 is characterised in that the perimetral ring 4 is actuated by means of hydraulic cylinders 51 actuated by a hydraulic circuit 7 capable of controlling the pressure in all process steps. Such a hydraulic circuit 7, which further comprises a valve assembly 71, 73, 72, is integrated into mold 1.

Mold 1 comprises movement means 5 of the perimetral ring 4 adapted to carry out the advancement and retraction of the ring within the mold, hereafter referred to as ring movement means.

The ring movement means 5 comprise a hydraulic circuit 7 (shown in FIG. 8) provided with actuating cylinders 51 activated/deactivated by hydraulic controls commonly found in injection presses (and referred to as radial). Usually, and disadvantageously, radials can only be used in standard presses when the mold is open.

In particular, the hydraulic circuit 7 comprises a first hydraulic control 91 (referred to as radial one) and a second hydraulic control 92 (referred to as radial two). The radial or hydraulic control works with the following logic: if the hydraulic control is ON, the pressurised oil provides a constant pressure equal to the set pressure; if the control is OFF, the oil in the hydraulic circuit is free to flow towards a tank at atmospheric pressure. This last step is called ‘exhaust pressure’.

The hydraulic circuit 7 for moving ring 5 is thus connected and managed directly by the hydraulic control 91 (or radial one) of the press.

The hydraulic circuit 7 comprises a first non-return valve 71, of the type with release controlled by pressure, directly controlled by radial two 92.

The hydraulic circuit 7 further comprises a pressure relief valve 73, which allows setting the pressures of the circuit and which can be calibrated between zero and the pressure of radial one.

The hydraulic circuit 7 further comprises a second non-return valve 72.

The hydraulic circuit further comprises a pressure gauge 76, to which a valve 75 is associated, for pressure control.

The hydraulic circuit 7 comprises a series of T-unions 74.

The ring movement means 5 comprise a plate 50 which can be lifted by means of the actuating cylinders 51 of the ancillary hydraulic circuit. Plate 50 is provided with axial pins 52 connected to the perimetral ring 4.

Plate 50, also referred to as “table”, is intended to ensure the coordination and parallelism of the movement (lifting and retraction) of pins 52 and thus, of the perimetral ring 4.

Plate 50 is provided with a constant preload, provided by the actuating cylinders 51 of the hydraulic circuit 51 by means of the pressure relief valve 73.

Advantageously, the hydraulic circuit 7 is integrated in mold 1 and is connected to the first hydraulic control 91 (referred to as radial one) and to the second hydraulic control 92 (referred to as radial two). This solution allows ensuring both the necessary seal of the closure and the absence of residual forces when opening mold 1, thus avoiding the ejection of the piece at the same time as the opening.

FIGS. 1 to 7 show the operating diagram of mold 1 and of the perimetral ring 4, moved by the ring movement means 5 and by the hydraulic circuit 7.

FIG. 5 shows the compression step with completely closed mold, i.e. with matrix 3 closed on punch 2. The perimetral ring 4, even if completely retracted due to the thrust of matrix 3, pushes on piece P: the actuating cylinders 51 of hydraulic circuit 7 are active and at constant pressure given by the pressure relief valve 73.

Once the cooling step has been completed, the piece is ready. From this moment, the radial controls (radial one and radial two) of the machine can be used. Radial two is pressurised: acting on the non-return valve 71 it causes the release of pressure of the hydraulic circuit 7: the actuating cylinders 51 drop to zero pressure and the perimetral ring 4 stops pushing on the piece.

FIG. 6 shows the opening step of the open mold, i. e. matrix 3 raised with respect to punch 2, and the perimetral ring 4 in the retracted and at rest position (i.e. not in thrust). The actuating cylinders 51, having lost pressure, do not push the piece during the opening step of mold 1 so that it does not fall out before the picking robot arrives. The extraction of the piece is carried out by means of special extractors (not shown), while the pressure in the hydraulic circuit 7 for the movement of the perimetral ring 4 remains at zero until the extraction has taken place.

FIG. 1 shows the mold still open but the perimetral ring 4 is now in the advanced position (or in thrust) for a new molding cycle. Radial one, connected directly to the ring movement circuit 5, is put under pressure and the perimetral ring 4 moves forward. At this point, the pressure in radial one is reset to zero (and it could not be otherwise in a standard press), while in the hydraulic circuit 7, due to the non-return valves 71, 72, pressure remains to the value set in pressure relief valve 73 (which can be calibrated between zero and the pressure of radial one).

In FIG. 2, mold 1 closes and the perimetral ring 4, pushed forward by the actuating cylinders, 51, retracts under the thrust of matrix 3. The retraction of the perimetral ring 4 reduces the volume of oil in the actuating cylinders 51, and in the hydraulic circuit 7 the constant pressure given by the pressure relief valve 73 remains.

At this point, the injection-compression step begins.

FIG. 3 shows the injection step of the molten material into the injection chamber 10.

FIG. 4 shows the compression step of the material. Mold 1 resumes closing, squeezing the molten material inside the injection chamber 10. As shown in FIG. 8, the molten material is pushed towards the edges of the injection chamber 10 when mold 1 is still open, but its travel is stopped by the perimetral ring 4 which is still in abutment between matrix 3 and punch 2 to closure mold 1, due to the force determined by the pressure relief valve 73.

FIG. 5 shows the compression step end in which the mold is completely closed (matrix 3 in abutment on punch 2). At this point the molded part is cooled, with the mold still closed, and the cycle resumes.

As can be understood from the description, mold 1 according to the invention allows overcoming the drawbacks of the prior art.

Advantageously, the particular geometry of the perimetral ring prevents the pressure of the molten material from pushing the ring itself outwards of the mold, ensuring the proper support of the perimetral ring during the injection of the material into the mold.

Advantageously, moreover, the particular position of the perimetral ring closing the mold prevents the formation of triple junction points, and it effectively hides the junction lines on the piece.

Advantageously, the perfect control of the perimetral ring when opening the mold prevents the ejection of the piece at the same time as the opening of the mold without risk of damage to the piece surface, even with a press of the type commonly available on the market.

A man skilled in the art may make several changes and adjustments to the molds and molding methods described above in order to meet specific and incidental needs, all falling within the scope of protection defined in the following claims. 

1. A mold for injection-compression molding comprising: a punch; a matrix suitable to close on the punch so as to delimit with the punch an injection chamber to contain material to be injected, said matrix and punch being axially movable relative to one another with respect to an axial direction of opening or closing of the mold, a perimetral ring slidingly associated with the punch or the matrix, along the axial direction, suitable to define, together with the matrix and the punch, a profile of the injection chamber; wherein the perimetral ring comprises an interface profile, configured to engage with the matrix or with the punch, equipped with a geometry suitable to generate in the closing phase of the mold, a compression force of the ring towards an inside of the mold.
 2. The mold according to claim 1, wherein the interface profile is convex.
 3. The mold according to claim 1, wherein the interface profile comprises an upper inclined thrust wall, the upper inclined thrust wall engaged by an inclined plane associated with the matrix, so as to generate a force substantially transverse with respect to the axial direction and with a direction towards the inside of the mold.
 4. The mold according to claim 1, wherein, in closing the mold, the perimetral ring is interposed between the matrix and the punch, thereby creating double junction points on the piece.
 5. The mold according to claim 4, wherein the perimetral ring defines together with the punch, a double junction point in correspondence of a thickness of the piece.
 6. The mold according to claim 4, wherein the perimetral ring defines, together with the matrix, a double junction point in correspondence of an outer edge of the piece.
 7. The mold according to claim 1, wherein the perimetral ring defines, together with the matrix and the punch, the profile of the molded piece.
 8. The mold according to claim 7, wherein the perimetral ring defines, together with the punch, a thickness of the piece in such a way as to hide junction lines on the piece.
 9. The mold according to claim 1, wherein the interface profile is convex polygonal.
 10. Mold, accordance to claim 1, comprising an integrated hydraulic circuit for advancement and retraction of the perimetral ring.
 11. The mold according to claim 10, wherein the hydraulic circuit comprises hydraulic cylinders activated and/or deactivated by at least one first hydraulic control and at least one non-return valve.
 12. The mold according to claim 10, wherein the hydraulic circuit comprises at least one first non-return valve directly controlled by a second hydraulic control and a second non-return valve.
 13. The mold according to claim 10, wherein the hydraulic circuit comprises a pressure relief valve allowing setting pressures of the circuit between zero and the pressure of a radial control.
 14. The mold according to claim 10, wherein the hydraulic circuit comprises a pressure gauge for controlling the pressure.
 15. A method of closing of a mold according to claim 10, comprising the step of: opening the mold with the perimetral ring in an advanced position and in thrust: switching on a first hydraulic control, connected directly to the hydraulic circuit that is put under pressure and determining advancement of the perimetral ring; switching off the first hydraulic control, in which the pressure is returned to zero, while in the hydraulic circuit, due to the non-return valves, the pressure remains at a value set in the pressure relief valve; closing the mold with perimetral ring in thrust with a force given by the pressure set with the pressure relief valve; injecting molten material in the injection chamber; compressing the material in the injection chamber up to complete closure of the mold; cooling of the molded piece with the mold closed: switching on a second hydraulic control, that is put under pressure and, by acting on the non-return valve, determines release of the pressure of the hydraulic circuit, which drops to zero and the perimetral ring stops pushing on the piece; opening the mold with perimetral ring in a retracted position and rest position.
 16. The method of closing a mold according to claim 15, further comprising the step of: extracting the piece using an extractor, while the pressure in the hydraulic circuit remains at zero until after extraction. 